An interesting read on parasitic loss from mechanical fans:

Dyno testing on fans, alternators and oil.....all here!

A couple of issues to note if reading it:

1) The fan/clutch systems on the older Chevy Smallblock is going to be quite a bit larger/heavier than on the X5s.

2) Fan draw on a stationary dyno will differ than while driving (be larger) due to frontal flow in proportion to the speed you're traveling.

However, it gives a little insight on the subject with measured data.

I personally wouldn't remove my mechanical fan simply for the sake of performance gains on something like the X5, however, the other aspects like being able to have control over what temperature it starts/stops, being less of a PITA to work on the car, etc. is why I would switch it over like my E39 and Wrangler should the motivation/opportunity arise.

I wouldn't suggest it be a "go to" modification, but like all modifications to a car, whether it is worth it or not depends on your own desires/needs.